low voltage lighting: higher current?

[Andrewh01]

I am just starting to learn all about electricity - I have learnt alot recently.

I did Physics in school so I learnt all the forumlas etc etc, its a shame I only paid a bit of attention to it! Alot is coming back though.

A question about low voltage lights - If the voltage is lowered, then the current will increase - correct?

Isnt higher current more dangerous?

I remember learning about coulombs of charge. What is the idea/formula that shows that 240v is more potent than 24v - doesnt that relate to the coulombs of chare??

I was looking at the power box in the pool shed and noticed there is an 80A (!!) fuse, which is for the two pool lights. In this case, isnt that extremely high & dangerous ??? Or would the voltage be lower, hence the higher current??

 

[fido]

Yes, low voltage circuits need thicker cables, heavier duty switches etc. but are safer when it comes to electric shocks due to the high resistance of human skin, meaning very little current can actually pass through you if you come into contact with a low voltage. 12 volt car wiring is just as capable of going up in flames as 240 volt house wiring if there's a short circuit though.

 

[Andrewh01]

Thanks for that reply.

So going back to the pool shed - Im pretty sure the power that comes to the pool shed is 240V (im in australia). It is in thick conduit, probably 1 1/2in" thick.

The pool lights are 32V. The circuit breaker for the lights is 80A. Wouldnt that mean the cable to the pool lights would be really thick ???? Also, wouldnt it mean the lights themselves are quite power hungry?? (somewhere around 2000W????).

Sorry if this sounds very newbie ... ! Have to start somewhere!

 

[slippyr4]

A question about low voltage lights - If the voltage is lowered, then the current will increase - correct?

yep

Isnt higher current more dangerous?

It is, if that current is flowing through you.

But, as you will remember, Current Voltage and Resistance are interrelated. Remeber V=IR ? Given a constant resistance, V is proportional to I, therefore, lower volts mean lower current through that fixed resistance (ie, your body). So, lower voltages develop lower currents.

"low voltage" (properly Extra Low Voltage) lightbulbs which are rated at equivalent power outputs to the 230V ones have lower resistances, therefore a greater current is induced. If you were to put a 12V 50W bulb into a 230V supply, it'd pull nearly 80 A, and produce 18,000 W of light/heat. Although this would happen for mere microseconds before the bulb blew.

I remember learning about coulombs of charge. What is the idea/formula that shows that 240v is more potent than 24v - doesnt that relate to the coulombs of chare??

Current is the flow of charge - an Amp is defined as one coulomb per second. Voltage, on the other hand, describes potential.The volt is defined as one joule per couloumb, it's how much energy potential a couloumb has.

If you extend that thought a little, you can see that multiply volts (joules per coulomb) by amps (coulombs per second), you end up with joules per second, which is watts. And you can see that volts kind of say how energetic the coulombs are, and amps say how many coulombs there are.

I was looking at the power box in the pool shed and noticed there is an 80A (!!) fuse, which is for the two pool lights.

Thats unlikely, you're probably looking at the RCD, which will have two ratings - something like 80A / 30ma - 80 A is simply the maximum current it can safely switch, and the 30ma is the leakage current that it will switch at. RCD's are not overcurrent devices - you should have MCB's or fuses in there as well which will provide the overcurrent protection.

 

[1974stephen]

The main thing I remember from a lecturer I had at college when talking about voltage and ampage, was that..... "volts jolt, but amps kill!"

 

[plugwash]

the expression volts jolts mills kills is often repeated but highly misleading, what matters is tissue current, route through the body (through the heart is BAD), duration (thats why rcds are usefull they can stop a shock in a very short time) and sometimes frequencey (DC can be nasty because it can freeze muscles, rf has its own special ways of being nasty). Since a persons body won't draw anywhere near enough current to cause significant voltage drop on a mains line the two factors determining tissue current are the voltage and the resistance of the body, the current rating of the cuircuit is basically irrelevent.